1. Field of the Invention
This invention relates generally to a hydraulically actuated control element of an automatic transmission, such as a clutch or brake, having a balance dam.
2. Description of the Prior Art
A balance dam is a device used in a hydraulically actuated clutch to reduce differential pressure across a piston of an actuating servo induced by centrifugal force.
Many balance dams include a vent near the inside diameter of the balance piston. This vent is intended to both allow oil to exit the balance dam when the clutch-apply piston strokes to engage a clutch and also to reduce the pressure in the balance dam cavity during normal operation. However, the balance cavity dam will at least partially empty through the vent when the clutch is stationary and when the engine is not running.
After the balance dam is partially drained and upon restarting the engine, the balance dam does not contain enough oil to function properly. If a vehicle operator immediately depresses the accelerator pedal and increases engine speed, this action may cause the clutch apply piston to inadvertently drift, thereby increasing torque capacity of the clutch.
In some transmissions while the engine rotates at idle speed, a clutch will rotate at a ratio of transmission input shaft speed, which will rotate at a ratio of engine speed.
This problem is further compounded by the frequent practice of applying other drive clutches when the transmission is in the neutral operating range. This allows the possibility of transferring torque to the output shaft, if engine speed is increased immediately after starting the engine and before the balance dam is refilled with fluid.